1. Field of Art
This invention relates to apparatus for running wireline, coiled tubing or other small diameter conduit inside of a larger tubular string, such as conventional drill pipe. With further particularity, this invention relates to an entry sub for pressure-controlled access to the interior of a drill string with a top drive unit in place, in which the apparatus is connected linearly with the drill string below a top drive unit, the wireline enters a seal assembly in the entry sub substantially aligned with the center axis of the drill string, the lift axis of the entry sub is concentric with the center axis of the drill string, and full pumping capability is retained through the top drive and drill string.
2. Related Art
At times, in the course of drilling oil and gas wells, it is necessary to run certain tools down through the inside of the drill string, with the drill string extending downhole. In this description, it is understood that the term "drill string" means, without limitation, the string of tubular members employed in drilling oil and gas wells, usually including drill pipe, drill collars, and possibly other downhole tools. Similar strings may be used in workover procedures as well.
One such situation is when a drill string becomes stuck during drilling operations. Diagnostic tools, commonly called "free point" tools, are run downhole within the bore of the drill string. Through manipulation of the drill string with the free point tool in place, the depth at which the drill string is stuck can be determined. Once the stuck point is determined, an explosive charge or "string shot" is run to a point above the stuck point, then discharged, loosening a drill string connection downhole and permitting the drill string to be backed off and a portion removed from the hole.
Other types of thru-drill string tools that are run on wireline include formation evaluation logging tools. In addition to wireline, it is occasionally desired to run coiled tubing downhole through the drill string. Although the following description discusses the running of wireline, it is understood that the discussion applies equally to other small diameter conduits such as, by way of example, coiled tubing.
Various tools have been employed to permit running small diameter conduits within a drill string, while retaining pressure control over the drill string. When running of such small diameter conduits, including wireline (whether "slickline" or electric line) and coiled tubing, the operational goals which such tools should satisfy include:
1) routing the wireline (or other small diameter conduit) from a spool or drum that is axially displaced from the center axis of the drill string, to a position substantially coincident with the center axis of the drill string, so that the wireline may run downhole with minimal rubbing contact on the interior of the tools and drill string (wearing both the drill string and the wireline); PA1 2) maintaining a pressure seal about the wireline and thereby maintaining pressure control over the drill string; PA1 3) preserving fluid pumping capability through existing rig equipment (for example a top drive unit or conventional drilling swivel), down through the drill string; and PA1 4) permitting linear and rotary manipulation of the drill string without creating undesirable torque or bending moment forces in the drill string. PA1 a) to provide an entry sub which permits introduction of small diameter conduit, such as wireline or coiled tubing, into a drill string; PA1 b) to provide an entry sub which retains pressure control over the drill string while small diameter conduit is worked into or out of the drill string; PA1 c) to provide an entry sub which aligns the wireline with the center axis of the drill string prior to passage of the wireline into the entry sub, minimizing wear on the wireline and the entry sub which may be caused by the wireline bearing upon the interior of the entry sub; PA1 d) to provide an entry sub which may be employed in conjunction with top drive units or conventional rotary drilling equipment, by being made up into the drill string below the top drive unit; PA1 e) to provide an entry sub which may be used in conjunction with different numbers of sheaves, depending upon the specific operational situation at hand; and PA1 f) to provide an entry sub which has a lift axis which is substantially aligned with the center axis of the drill string to which it is connected, for prevention of bending moments and the resulting torque and canting.
Various methods and apparatus exist in the related art for permitting running of wireline downhole within a drill string, while achieving perhaps some of the operational goals. Perhaps the oldest and simplest device is a "packoff assembly" which comprises a rubber sleeve with a hole therethrough, through which the wireline runs. The rubber sleeve is contained within a generally bowl-shaped body. A compression sleeve atop the rubber sleeve is threadably engaged with the bowl-shaped body, and is tightened to compress the rubber sleeve, in turn squeezing the rubber sleeve around the wireline and effecting a pressure seal. Other embodiments of the packoff assembly utilize a hydraulic actuated compression sleeve, which is forced downward onto the rubber sleeve by hydraulic force. Such packoff assemblies are typically made up in the uppermost box connection of a string of drill pipe, at a position above the rotary table. A number of drawbacks accompany such devices. The pressure capability is limited. Usually, a sheave must be mounted in the rig mast and aligned with the center axis of the drill string, with the wireline routed from a spool over that sheave and thence into the drill string. The hanging of the sheave is often problematic. With some packoff assemblies, it is not possible to maintain fluid pumping capabilities through the drill string, and those packoff assemblies that do provide for pumping usually employ a hose connected to a T-connection in the body of the packoff assembly, rather than being able to pump through the rig equipment (top drive and drill string). Significant restraints on fluid pressures and rates exist with such arrangements.
As a response to the significant limitations of the conventional packoff assembly, "entry subs" were developed. Two examples are disclosed in U.S. Pat. No. 4,681,162 to Boyd (Jul. 21, 1987) and U.S. Pat. No. 5,284,210 to Helms et al (Feb. 8, 1994). Both entry subs comprise a tubular member having a central body with lower threads for connection to the drill string. The central body has a bore which divides into first and second passages in its upper section, generally in a Y-shaped configuration. The first passage provides for threadable connection to the drill string above the entry sub (which may comprise a top drive unit or the like), while the second passage provides a means for introducing wireline, coiled tubing, or other small diameter conduit into the bore of the central body. A threaded connection at the upper end of the second passage provides a means for installing a pressure seal device around the wireline.
A common drawback to the entry subs disclosed in the Boyd and Helms et al references is that the wireline enters the entry sub at an angle to the center axis of the central body bore and of the drill string. As a result, the wireline bears against the low side of the interior of the entry sub, especially where the second "wireline" passage merges with the bore of the central body (in effect, where the wireline "turns the corner" from the angled second passage into the bore of the central body). Inevitably, wear occurs to both the wireline and the interior of the entry sub, and possibly to the drill string itself; recognizing such inevitable wear, both the Boyd and Helms et al entry subs have a replaceable section at the lower end of the entry sub where the majority of the wireline wear is expected.
Although an alternative embodiment in the Helms et al reference (FIG. 3 of the Helms et al '210 patent) provides a "straight line" entry of the wireline into the entry sub, such straight line wireline entry is accomplished at the expense of having an offset lift axis; that is, the drill string connection at the upper end of the Helms et al entry sub, in that embodiment, is offset from the center axis of the drill string. Canting of the entry sub and drill string results from the torque and bending moment generated with an offset lift axis. It is to be noted that both embodiments of the Helms et al entry sub have a lift axis which is not coincident with the center axis of the drill string; therefore, due to creation of a bending moment, torque and canting of the drill string inevitably occurs.
Wittrisch, U.S. Pat. No. 4,913,227 (Apr. 3, 1990) discloses another entry sub which provides a pair of guide rollers to align the wireline with the center axis of the drill string. However, Wittrisch does not permit pumping through the entry sub via the top drive or drill string. Rather, a side opening is provided in the sub below the seal assembly, for connection to a "pumping installation" (which would necessarily comprise hoses or the like, with attendant rig-up problems, pressure limitations and the like).